1. Field of the Invention
The invention generally relates to an illumination system of a microlithographic projection exposure apparatus, and in particular to an apparatus comprising an array of micromirrors or other beam deflecting elements that can be individually controlled.
2. Description of Related Art
Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to light of a certain wavelength. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be imaged onto the photoresist. After exposure the photoresist is developed to produce an image that corresponds to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system that illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example. The apparatus further comprises a mask stage for aligning the mask, a projection objective (sometimes also referred to as ‘the lens’) that images the illuminated field on the mask onto the photoresist, and a wafer alignment stage for aligning the wafer coated with the photoresist.
One of the essential aims in the development of projection exposure apparatus is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to a high integration density, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus.
Various approaches have been pursued in the past to achieve this aim. One approach is to improve the illumination of the mask. Ideally, the illumination system of a projection exposure apparatus illuminates each point of the field illuminated on the mask with projection light having a well defined spatial and angular irradiance distribution. The term angular irradiance distribution describes how the total light energy of a light bundle, which converges towards a particular point on the mask, is distributed among the various directions of the rays that constitute the light bundle.
The angular irradiance distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be imaged onto the photoresist. For example, relatively large sized features may require a different angular irradiance distribution than small sized features. The most commonly used angular irradiance distributions are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil surface of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil surface. Thus there is only a small range of angles present in the angular irradiance distribution of the projection light, and all light rays impinge obliquely with similar angles onto the mask.
Different approaches are known in the art to modify the angular irradiance distribution of the projection light in the mask plane so as to achieve the desired illumination setting. For achieving maximum flexibility in producing different angular irradiance distribution in the mask plane, it has been proposed to use mirror arrays that determine the irradiance distribution in the pupil surface.
In EP 1 262 836 A1 the mirror array is realized as a micro-electromechanical system (MEMS) comprising more than 1000 microscopic mirrors. Each of the mirrors can be tilted about two orthogonal tilt axes. Thus radiation incident on such a mirror device can be reflected into almost any desired direction of a hemisphere. A condenser lens arranged between the mirror array and a pupil surface translates the reflection angles produced by the mirrors into locations in the pupil surface. This illumination system makes it possible to illuminate the pupil surface with a plurality of spots, wherein each spot is associated with one particular mirror and is freely movable across the pupil surface by tilting this mirror.
Similar illumination systems using mirror arrays are known from US 2006/0087634 A1, U.S. Pat. No. 7,061,582 B2 and WO 2005/026843 A2.
The mirror array, or a microlens array that is used to direct individual light beams onto the mirrors of the array so as to ensure that no light is lost in gaps formed between adjacent mirrors, should be illuminated by the projection light uniformly or with only moderate irradiance gradients. It is also important that transient variations of the projection light beam, which are produced by the light source of the illumination system, do not have any impact on the performance of the illumination system. For example, with excimer lasers, which are often used as light sources, effects are observed which are referred to as laser pointing or laser jitter. These effects manifest themselves as small variations of the direction of the projection light beam emerging from the light source.
In order to prevent that such variations have an adverse impact on the illumination of the mask, WO 2009/080279 A1 proposes to arrange a beam homogenizing unit between the light source and the mirror array or a microlens array placed in front of the mirror array. The beam homogenizing unit comprises an optical integrator, which includes a first and a second optical raster plate, and a condenser having a front focal plane in which the second optical raster plate of the optical integrator is arranged. Since the first optical raster plate is arranged in the front focal plane of the lenses of the second optical raster plate, this front focal plane is imaged on the back focal plane of the condenser. The angular light distribution at the first optical raster plate therefore has no impact on the spatial irradiance distribution in the back focal plane of the condenser. Since the images of the object planes of the lenses of the second optical raster plate superimpose in the back focal plane of the condenser, shifts of the irradiance distribution on the first optical raster plate only substantially affect the angular, but not the spatial light distribution in the back focal plane of the condenser.
However, an optical integrator necessarily increases the geometrical optical flux of the projection light. Thus the divergence of the projection light behind the optical integrator is greater than in front of it. However, for the beam homogenizing unit an increase of the divergence is not desired, because the divergence shall be increased only by the mirror array. The larger the divergence of the projection light is when it impinges on the mirror array, the larger will be the spots that are produced by the mirrors of the array in a subsequent pupil plane. But only with very small spots it is possible to produce arbitrary angular light distributions in the mask plane.
The increase of the divergence, which is produced by optical integrators if the input divergence does not exceed certain limits, can be kept small if the lenses of the second optical raster plate have a small refractive power, which implies large focal lengths of the lenses. However, a large focal length requires a large distance between the optical raster plates and also between the optical integrator and the condenser. This problem can be solved by the use of folding mirrors. However, if the divergence of the projection light is small, optical crosstalk between adjacent channels of the optical integrator produced by diffracted light becomes an issue. In conventional optical integrators which are designed to increase the divergence significantly, the distance between the optical raster plates is so small that light, which is diffracted by the regular arrangement of lenses, will remain confined within the respective channel of the optical integrator. However, if the divergence is small and the distance between the optical raster plates becomes large, diffracted light may enter adjacent channels and contribute to optical crosstalk.
Optical crosstalk in optical integrators modifies the irradiance distribution in the back focal plane of the condenser of the beam homogenizing unit. The main problem is that the optical crosstalk usually changes if the direction of the impinging light beam is not stable. Then laser pointing or other transient disturbances may change the spatial irradiance distribution in the back focal plane of the condenser of the beam homogenizing unit. This will ultimately change the angular light distribution in the mask plane. In other words, optical crosstalk destroys the property of the beam homogenizing unit to produce a superimposed spatial irradiance distribution which is substantially independent of the angular distribution of the light impinging on the optical integrator.